In a fundamental study on the early detection of lung cancer by helical computed tomography (CT), we evaluated the effect of scanning parameters on radiation dose and image quality using a phantom. The exposure dose at the center of the gantry and the absorbed dose at the center of an acrylic phantom changed linearly in proportion to tube current. On the other hand, as the table feed speed increased, the rate of reduction in these doses became smaller. While the change in CT value of an acrylic sphere in the longitudinal axis, the difference in CT value between an acrylic sphere and styrofoam in the horizontal plane and the CT profile curve of an acrylic sphere in the horizontal plane did not change according to tube current, an increase in table feed speed brought about the degradation of these items. In conclusion, reduction of the tube current in helical scanning is able to reduce radiation dose without causing a significant change in image quality, but an excessive increase in table feed speed does not reduce radiation dose effectively and is associated with degradation in image quality.